Abstract: A mandrel for bending a rectangular duct body is provided. A round part and a shaft pin hole are formed at the front end of the mandrel, connection pieces and rollers are formed at the front end of the mandrel, protruded parts for supporting the bent part of the rectangular duct body and depressed parts in which depression parts will be formed are formed on both sides of the rollers, and oil grooves are formed in the sides of the rollers. Accordingly, there is an advantage in that workability and productivity can be improved because frictional force between the rollers and between the inside of the rectangular duct body and the rollers is reduced when performing a bending work because grease within the oil grooves formed in the rollers performs a lubrication action.